You will need to file two state returns this year, you will file a state return for Montana as you both lived there and earned income. Montana Information
For the State of Illinois, your wife only will need to file as a Part Year Resident for the State of Illinois. Illinois Information
Your spouse can file a Part Year DC Illinois Married filing separate on the same return. For your dates of residency, leave blank. You will be able to use the federal tax return but allocate the income and deductions to reflect only her Illinois income and deductions.
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